During normal operation, various conditions may cause a power supply to experience dangerously increased output voltages and currents. In such a dangerous situation, the overload can severely damage or destroy electronic components or even the entire power supply. In response, most switching and linear power supplies possess output safeguards to protect power supplies from voltage and current overloads. Basically, external electronic circuits must sense the output of the power supply and react accordingly. In cases of above normal currents, circuits must limit the output current to a certain acceptable level or shut down the power supply in case of short circuit or heavy overload. Further, circuits monitor the output voltage and disable the power supply when necessary.
For those independent functions, current limiting (light overload as part of normal operation) and short-circuit (heavy overload) protection, multiple functional circuits are needed. Usually, current limiting is performed with operational amplifiers to maintain maximum output current (i.e., a first current threshold). A current limiting circuit is usually coupled to the power supply and merges into the output voltage feedback control loop. Once the output current rises above the first current threshold, the circuit lowers the output current by lowering the output voltage through a controller circuit.
If, for any reason, output current of the power supply goes high enough to damage the supply, prudence stipulates automatic power supply shut-down. Such a shut-down function engages when the output current rises above a previously set maximum overload current (i.e., a second current threshold). Upon sensing the unacceptable rise in the output current by a comparator, the independent shut-down circuit disables the power supply.
Additionally, overvoltage protection is employed for the power supply output to prevent the destruction of the supplied equipment do to malfunction of the power supply itself. As an independent function, the overvoltage protection requires a separate protection circuit. This protection is usually accomplished by a comparator set to trigger at a slightly higher than nominal output voltage (i.e., 15%-20% higher than the nominal output voltage). In cases of over-voltage, power supply is either shut-down and allowed to restart in a "hiccup" mode, or is latched until the input power is turned off and turn-on again, depending on the particular application.
Discrete or integrated controllers of linear or switching power supplies, usually implement some of the aforementioned protection features. In common switching power supplies, a feedback controller integrated circuit contains an internal overvoltage protection comparator, set to 115%-120% of the nominal output voltage. Therefore, when the internal overvoltage protection circuit senses overvoltage, the comparator will shut-down the power supply and restart it in a "hiccup" mode. Therefore, for the overvoltage protection, output current limiting and short-circuit current protection, multiple functional independent circuits must be added externally to the power supply.
Accordingly, what is needed in the art is circuit that functions to limit a power supply output current or shut down the power supply under short-circuit conditions.